Hudson provides machine-consumable remote access API to its functionalities. Currently it comes in three flavors:

XML

JSON with JSONP support

Python

Remote access API is offered in a REST-like style. That is, there is no single entry point for all features, and instead they are available under the ".../api/" URL where "..." portion is the data that it acts on.

Sample code

XPath selection

The XML API supports a selection by XPath by using the query parameter 'xpath'. This is convenient for extracting information in environments where XML manipulation is tedious (such as shell script.) See issue #626 for an example of how to use this.
See .../api/ on your Hudson server for more up-to-date details.

XPath exclusion

Similar to the 'xpath' query parameter above, you can use (possibly multiple) 'exclude' query patterns to exclude nodes from the resulting XML. All the nodes that match the specified XPath will be removed from the XML.
See .../api/ on your Hudson server for more up-to-date details.

Depth control

Sometimes the remote API doesn't give you enough information in one call. For example, if you'd like to find out all the last successful build of a given view, you'd realize that the invocation to the remote API of the view won't give you this, and you'd have to recursively call the remote API of each project to find this out. The depth control, introduced in 1.167, solves this problem. To understand this feature, it's good to start with how the remote API works.

The data model that Hudson maintains internally can be thought of as a big tree structure, and when you make a remote API call, you are getting a small subtree of it. The subtree is rooted at the object for which you made a remote API call, and the sub-tree is cut beyond certain depth to avoid returning too much data. You can adjust this cut-off behavior by specifying the depth query parameter. When you specify a positive depth value, the subtree cut-off happens that much later.

So the net result is, if you specify a bigger depth value, you'll see that the remote API will now return more data. Because of the algorithm, this works in such a way that the data returned by a bigger depth value includes all the data returned by smaller
depth value.

See .../api/ on your Hudson server for more up-to-date details.

Detecting Hudson version

To check the version of Hudson, load the top page and check for the "X-Hudson" response header. This contains the version number of Hudson, like "1.358" This is also a good way to check if an URL is a Hudson URL.

Discovering Hudson on the network

Hudson instances listen on UDP port 33848. Whenever a UDP packet is received, it will respond with a short XML fragment that shows the connection information. This XML has the following format:

<hudson>
<version>1.380</version> <!-- version of Hudson -->
<url>http://somwhere/hudson/</url> <!-- HTTP URL. Not available if not configured -->
<slave-port>12345</slave-port> <!-- if TCP slave listener port is configured, its number -->
</hudson>

By using this, a client can use a UDP broadcast to try to discover nearby Hudson instances. This is primarily useful for a self-organizing build cluster.

Is there any way to use this API to tell Hudson to "keep this build forever" for...

Is there any way to use this API to tell Hudson to "keep this build forever" for a particular build. I am working on some automation for publishing a particular build to an FTP site and this is one of the steps I would like performed. So far, I haven't been able to figure out a programmatic way to mark a given build as "keep" in Hudson. Ideas?

Replacing ViewNameHere and JobNameHere as appropriate and specifying the build number in place of 'nn'. Obvious after viewing the HTML, but it would be cool to have a page documenting this type of thing. Just an idea ... I sure don't have the time to do it 8-(

#!/usr/bin/env python
# this script is only demo purpose which is designed to get properties of job, queue, like
# nextBuildNumber. But note the logistics might not be correct
import urllib2
#call api of job 'git_plugin_test'
url="http://localhost:9001/job/git_plugin_test/api/python?depth=0"response=urllib2.urlopen(url)
build_dict=eval(response.read())
#call api of job 'queue' of hudson (global but not specific for one job)
url="http://localhost:9001/queue/api/python?depth=0"response=urllib2.urlopen(url)
queue_dict=eval(response.read())
print ''*40,'build dict',''*40
#print properties of job
for eachKey in build_dict:
print eachKey,build_dict[eachKey]
print
print ''*40,'queue dict',''*40
#look through items in queue and can be extended to forecast the job build
#number in for one item in queue
for index in range(1,len(queue_dict['items'])):
print ''*40,'queue hash',''*40
qi_action=queue_dict['items'][index]['actions']
list_para=qi_action[0]['parameters']
for index1 in range(0,len(list_para)):
print list_para[index1]
if list_para[index1]['name'] == 'SLEEP_TIME' and list_para[index1]['value'] == '62':
print "OK"
#only valid when no more than one build found in queue
if build_dict['inQueue']:
build_number=int(build_dict['nextBuildNumber']) + 1
else:
build_number=int(build_dict['nextBuildNumber'])
print "Hudson Build URL:",build_dict['url']+str(build_number)
print "Current build tree:"+build_dict['builds'][0]['url']